Insights into the source characterization, risk assessment and ozone formation sensitivity of ambient VOCs at an urban site in the Fenwei Plain, China

Abstract

The ground-level O₃ concentration has shown a deteriorating trend in the Fenwei Plain of China, which poses a greater challenge for the formulation of control strategies for O₃ precursor (VOCs). To accurately control VOCs sources and effectively reduce O₃ concentration from the aspect of season, online monitoring of 114 VOCs was conducted in Yuncheng Middle School Station from January 1, 2021 to December 31, 2021. The VOCs concentration showed a seasonal variation with the highest in winter and the lowest in summer. During the four seasons, alkanes (34.5-41.7%) and OVOCs (36.6-46.9%) were the most abundant species. The emission ratios of specific VOCs species indicated that vehicular exhaust, industrial source, and combustion were the major VOCs sources. The Positive Matrix Factorization (PMF) model identified that industrial source and secondary conversion were the main contributors in summer, while combustion and LPG/NG with the higher contribution in winter. The 2021-based VOCs emission inventory showed that the total VOCs emissions in the central urban area of Yuncheng was 8128.8 t, in which industrial process was the largest contributor. Alkanes, aromatics, and OVOCs accounted for 31.0%, 25.8%, and 25.7% of the annual VOCs emission, respectively. In addition, the calculated relative incremental reactivity (RIR) values of O₃ precursors demonstrated alkenes and aromatics were the most sensitive groups to O₃ formation during the four seasons. The ambient VOCs posed the non-carcinogenic risk in all seasons, which can be attributed to acrolein and three main sources (industrial source, secondary conversion, and combustion). However, ambient VOCs were exposed to definite carcinogenic risks due to the appearance of 1,2-dichloroethane, 1,2-dichloropropane, and benzene, and the main risks arose from industrial source, vehicular exhaust, and solvent usage. These findings emphasize the necessity of undertaking scientific and systematic measures for priority species and control sources of VOCs emission.